udy of g-Mg3N2 as an anodematerial for Na-, K-, Mg-, Ca- and Al-ion storage†
Lixin Xiong,a Hewen Wang,*b Wan Xiong,a Shicheng Yua and Chuying Ouyang *a
Searching for electrodematerials for non-lithiummetal ion batteries (NLMIBs) is key to the success of NLMIBs.
In this work, we investigated the scientific feasibility of using g-Mg3N2, which is a novel 2D graphene-like
material, as an anode for non-lithium metal-ions (Na, K, Mg, Ca and Al) batteries based on density
functional theory calculations. The sequential adsorption energy, Bader charge, intercalation voltage,
energy-storage capacity, electronic conductivity and metal-ion diffusion energy barrier are calculated.
Results show that the metal-ion intercalation potentials and diffusion energy barriers are suitable for battery
application. The maximum specific capacities for Na-, K-, Mg-, Ca- and Al-ion on g-Mg3N2 are predicted
to be 797, 797, 531, 1594 and 797 mA h g�1, respectively. The excellent structural stability of g-Mg3N2 is
good for the cycling performance. Moreover, the electronic structure of the g-Mg3N2 changes from
semiconductor to metal upon metal-ion adsorption, as well as relatively low metal-ion diffusion energy
barriers (except for Al-ion diffusion), are beneficial to the charge/discharge rate of the g-Mg3N2 anode.
1. Introduction
Developing sustainable and renewable energy to replace tradi-tional fossil fuels has always been a signicant challenge in theworld.1,2 The rapid development and application of wind andsolar energies has motivated researchers to look for novelenergy storage materials with larger capacities and longercycling life.3 Lithium-ion batteries (LIBs) as outstanding energystorage systems have been hugely successful in portable elec-tronic devices for decades.4 The excellent performance of LIBs,such as high working voltage and energy density, good cyclicstability and environmental friendliness, is exactly whatportable electronic devices and electric vehicles (EVs) need forpower supply.5,6 However, the inadequate natural abundance oflithium reserves, which in turn leads to expensive commercialcost, restricts the wide application of LIBs in large-scale elec-trical energy storage and EVs.7 Therefore, new metal-ionbatteries utilizing other elements such as Na,8 K,9 Mg,10 Ca11
and Al12 have been considered as potential alternatives to LIBs.These substitutes should possess low redox potentials, highspecic capacities, excellent cyclic stability and most impor-tantly, low cost.13
The key to the success of non-lithium metal-ion batteries(NLMIBs), which use non-lithium metal-ions as charge carrier,
is the properties of corresponding electrolyte and electrodematerials.14,15 Therefore, searching for new electrode materialswith appropriate performance becomes urgent from the scien-tic point of view. Recently, many cathodematerials for NLMIBshave been reported such as framework materials,16,17 polyanioncompounds18,19 and layered transition-metal oxides.20,21
However, due to the larger ionic radius and higher valence stateof non-Li metal-ions, the use of conventional LIBs anodematerials in NLMIBs is unsatisfactory.22,23 Fortunately, 2Dmaterials were found to have potential applications as anodesfor NLMIBs, both experimental and theoretically.24,25 Forexample, MXenes are considered as promising anode materialsfor NLMIBs due to their unique 2D structural character, whichbenets fast metal-ion diffusion and provides more space formetal-ion storage.26–28 Mortazavi et al. found that the at bor-ophene is promising anode material with superhigh capacitiesof Na- (1640 mA h g�1) and Mg-ion (2480 mA h g�1) storage.29
They also revealed the outstanding performance of boron-graphdiyne as an anode material with ultrahigh capacities of808 and 5174 mA h g�1 for Na- and Ca-ion storage.30 Lei et al.predicted that B2S monolayer is a promising anode material forNa- and K-ion batteries with high energy density and lowdiffusion barriers.31 Demiroglu et al. reported that the MXeneelectrode materials of Ti2CO2 and V2CO2 are promising for Na-ion battery applications, Ti2CO2 could be applied as low voltageapplications and V2CO2 is more appropriate for highervoltages.32
Recently, a novel 2D material g-Mg3N2 was predicted to bea stable graphene-like structure by a global particle-swarmoptimization method.33 It was found that the cohesive energydifference between the monolayer g-Mg3N2 and bulk phase
Mg3N2 is only 0.01 eV per atom, which leads to good chemicalstability of the g-Mg3N2 that is capable of withstandingtemperatures higher than 2000 K. The planar geometry of the g-Mg3N2 is highly graphene-like, and thus the advantages ofgraphene can also be found in the g-Mg3N2. In order to take thegraphene-like advantages of g-Mg3N2, it is worthwhile toinvestigate the scientic possibility of applying g-Mg3N2 as ananode material for NLMIBs.
First-principles calculations are proved to be a reliable tool tostudy the metal-ion adsorption, electrochemical potential,electronic conductivity and ionic diffusion in electrode mate-rials for NLMIBs.34–36 In this paper, we systematically investi-gated Na-, K-, Mg-, Ca- and Al-ion storage behaviors in g-Mg3N2
with rst-principles calculations. We begin our work fromsingle metal-ion adsorption on g-Mg3N2 in three particularsites, followed by a performance evaluation of g-Mg3N2 as ananode material for NLMIBs. The intercalation potential, theo-retical capacity, electronic structure, and metal-ion diffusionare studied. We nally conclude that g-Mg3N2 has potentialapplication as an excellent anode material for NLMIBs.
2. Computational method
In this paper, all calculations are implemented by the Vienna Abinitio Simulation Package (VASP) based on density functionaltheory (DFT).37 Projector augmented wave (PAW) method incombination with the generalized gradient approximation(GGA) expressed by the Perdew–Burke–Ernzerhof (PBE) func-tional are used.38,39 The cutoff energy for the plane-wave ischosen to be 550 eV for all calculations. van der Waals correc-tions are included via using the DFT-D3 method to betterdescribe the dispersion interactions between adsorbed metal-ions and host 2D materials.40,41 The lattice parameters as wellas the ionic positions are adequately optimized until the ulti-mate atomic forces and the internal stresses are converged to0.02 eV A�1 and 0.1 kbar, respectively. The density of states(DOS) are calculated by the Gaussian smearing method witha smearing width of 0.05 eV. The monolayer 2D host material isseparated by a 15 A vacuum layer along the z-axis direction. TheMonkhorst–Pack scheme 5 � 3 and 3� 3 k-point mesh are usedfor orthogonal unit cell and the 2 � 1 supercell, respectively.42
The nudged elastic band (NEB) method is applied to optimizethe metal-ions migration paths and evaluate the energybarriers.43 The charge distribution among metal-ions and 2Dhost is analyzed by the Bader charge analysis.44 The phonondispersion data are calculated using the linear responsemethod, and the phonon data is collected with the PHONOPYcode.45
Adsorption energies Ead are used to measure the metal-ionsadsorption strength on the 2D host material. Meanwhile, theamount of metal-ions storage on g-Mg3N2 can be evaluated bythe sequential adsorption energies dened as:
Ead ¼ Ehost+(n+1)M � Ehost+nM � EM (1)
where EM, Ehost+(n+1)M, and Ehost+nM are the calculated groundstate energies of the gas phase metal-atom in vacuum, and the
g-Mg3N2 host with (n + 1) and n metal-ions adsorbed, respec-tively. If the metal-ions adsorption energy is lower than thecohesive energy of the bulk phase metals, which are dened asthe total energy difference of one metal-atom in its bulk phasemetal and its gas phase state in vacuum, the intercalationpotential is positive. As the intercalation potential is alsodependent on the adsorption concentration, we need to calcu-late the sequential intercalation potential, which is dened as:
V ¼ �EMx2Mg3N2
� EMx1Mg3N2
� ðx2 � x1ÞEM
ðx2 � x1Þne ðx2 . x1Þ (2)
where EMx2Mg3N2, EMx1Mg3N2
, and EM are the total energies (in eV)of the Mx2Mg3N2, Mx1Mg3N2 and one metal atom in its bulkphase metals, respectively. x2 � x1 is the number of metal-ionsintercalated/deintercalated, and n refers to the correspondingvalence electrons of the metal-ion.46,47 The maximum theoret-ical capacity (CM) can be estimated by the following equation:
CM ¼ xmaxnF
MMg3N2
(3)
where xmax, n, F and MMg3N2are the maximum number of
adsorbed metal-ions, the number of corresponding metal-ion'svalence electrons, the Faraday constant and the mass ofMMg3N2
,respectively. The xmax is maximum number of metal-ions thatcan be adsorbed on the g-Mg3N2 before the potential decreasedto 0 V.
3. Results and discussions3.1 Single metal-ion adsorption on g-Mg3N2
The space group of g-Mg3N2 is P6/mmm (no. 191), with N atomsoccupying 3g Wyckoff position and Mg atoms occupyinganother 3g Wyckoff position, as shown in Fig. 1(a). The opti-mized lattice constants of primitive unit cell are a ¼ b ¼ 6.6316A, in good agreement with previous reports.33,48 In order tobetter demonstrate of the metal-ions adsorption sites, we usethe orthogonal unitcell that includes 6 Mg and 4 N atoms, asshown in Fig. 1(b). According to the symmetry of the g-Mg3N2 inthe orthogonal unitcell, only three sites are considered formetal-ions adsorptions. B-site is the bridge sites of N atoms(also top of the Mg atom), H-site is the center of hexagon ringand T-site is top of the N atom. Fig. 1(c–g) show the relaxedatomic structures with corresponding metal-ions adsorbed oneach site.
Table 1 presents the detailed information on energetics,structural and charge transfer of the g-Mg3N2 aer single metal-ion adsorption. Aer any metal-ion adsorption on B-sites (top ofMg atom), the corresponding Mg atom moved downwards andout of the graphene-like plane due to the repulsive interactionbetween them. As for different metal-ions adsorbed on the T-site, the negatively charged N atom and positively chargedmetal-ions attract each other and the N atom moves toward themetal-ions. Adsorption energies of different metal-ions in H-siteare higher than corresponding bulk phase metal cohesiveenergies, which are calculated to be �1.248, �0.999, �1.734,�2.066 and �3.678 eV for Na, K, Mg, Ca and Al, respectively.This shows that H-site is not suitable for metal-ion adsorption
Fig. 1 The top views of the atomic structure of the hexagonal g-Mg3N2 primitive cell (a) and the orthogonal g-Mg6N4 unitcell with metal-ions(black spheres) adsorption at B (bridge), T (top), and H (hollow) sites (b). (c)–(g) are the side views of the atomic structures with single metal-ionadsorption at B (up), H (middle), and T (down) sites after relaxation. The green, blue, yellow, purple, gray and red spheres denote Mg, N, Na, K, Caand Al atoms, respectively.
RSC Advances Paper
Ope
n A
cces
s A
rtic
le. P
ublis
hed
on 3
0 A
ugus
t 201
9. D
ownl
oade
d on
2/7
/202
2 6:
26:2
2 PM
. T
his
artic
le is
lice
nsed
und
er a
Cre
ativ
e C
omm
ons
Attr
ibut
ion-
Non
Com
mer
cial
3.0
Unp
orte
d L
icen
ce.
View Article Online
from energetics point of view, and thus H-site is no longerconsidered in later calculations. Meanwhile, the adsorptionenergies of different metal-ions in both B-site and T-site arethermodynamics favorable. The Bader charge analysis suggeststhat the adsorption energies are strongly correlated with thecharge loss of the metal-ions, namely, the more electrons metal-ions lose, the lower system energy is. This indicates theCoulomb interaction dominates the energetics of the metal-ionadsorption on g-Mg3N2. As Mg atom and metal-ions are posi-tively charged while N is negatively charged, metal-ions at T-sitehave the strongest attractive force from the closest N-atom(please refer to the distances of metal to Mg atoms dM–Mg
and N atoms dM–N given in Table 1). However, due to therepulsive forces from three nearest neighboring Mg atoms,adsorption energies at T-sites are higher than the correspond-ing metal-ion adsorption at B-sites. When metal-ions adsorbedat the B-site, there are two nearest neighboring N atomsattracting to them, as shown in Fig. 1(b). As for H-site, the N-distance is larger than the Mg-distance, leading to thestronger repulsive interaction and the weaker attractive inter-action. Consequently, all metal-ions adsorption energies at H-sites are high and not suitable for metal-ion storage.
3.2 Theoretical specic capacity and intercalation potential
Due to the repulsive interactions among positively chargedmetal-ions, the adsorption energies are also sensitive to the
Table 1 The transferred charges (Dq) frommetal-ions by Bader, the adsohost plane (height) and the distance from metal-ion to its nearest neigh
metal-ion adsorption concentration and increases with theincreased concentration. Meanwhile, in order to gure out themaximum amounts of metal-ions that can be adsorbed on the g-Mg3N2 surface before the adsorption energies reach the cohe-sive energy of the corresponding bulk phase metals, we calcu-lated the sequential adsorption energies at different metal-ionsconcentrations. We gradually add metal-ions according toabove single atom adsorption sites and energies until thesequential adsorption energies become close to the cohesiveenergies of the corresponding bulk phase metal. At the sameconcentration of metal-ions, we choose the adsorption cong-uration with the lowest energy. The adsorption energies asa function of the metal-ion adsorption concentration are pre-sented in the ESI (Fig. S1†), from which we can see that themaximum numbers of Na-, K-, Mg-, Ca- and Al-ions can beadsorbed on the unitcell of the g-Mg6N4 are 6, 6, 2, 6 and 2,respectively. The atomic structures and arrangement of theadsorbed metal-ions are presented in Fig. 2(a–e). The corre-sponding theoretical capacities for Na+, K+, Mg2+, Ca2+ and Al3+
are 797, 797, 531, 1594 and 797 mA h g�1, respectively.In addition to the theoretical capacity, voltage is another
important character of an electrode material. Since lower anodepotential is benecial for higher output voltage of the batterysystem, relatively low intercalation potential of an anodematerial is expected. The intercalation potentials on g-Mg3N2 asa function of their theoretical capacities are presented inFig. 2(f). As is shown, the obtained potentials are acceptable for
rption energies (Ead), the vertical distance from the metal-ion to the 2Dboring N/Mg atoms (Mg-, N-distance)
Fig. 2 Metal-ions adsorption configurations of maximum theoretical capacity are shown in (a)–(e), the green, blue, yellow, purple, gray and redspheres denote Mg, N, Na, K, Ca and Al atoms, respectively. The calculated sequential metal-ions intercalation potentials on g-Mg3N2 asa function of theoretical capacity are showed in (f).
Paper RSC Advances
Ope
n A
cces
s A
rtic
le. P
ublis
hed
on 3
0 A
ugus
t 201
9. D
ownl
oade
d on
2/7
/202
2 6:
26:2
2 PM
. T
his
artic
le is
lice
nsed
und
er a
Cre
ativ
e C
omm
ons
Attr
ibut
ion-
Non
Com
mer
cial
3.0
Unp
orte
d L
icen
ce.
View Article Online
NLMIBs anode applications. The theoretical capacity andvoltage are negatively correlated, which can be attributed to theweakened attractive electrostatic interaction between the metal-ions and the 2D host and the enhanced repulsive electrostaticinteractions among the metal-ions at a higher concentration.
As an anode material, the structural stability of the g-Mg3N2
is also very important, particularly aer the metal-ion isadsorbed (discharged states). In order to test the structural andthermal stability of the g-Mg3N2 anodes, phonon dispersioncurves calculated and AIMD simulations are performed. Thephonon dispersion curves of pristine g-Mg3N2 as well as Na- andMg-ion adsorbed cases (Na1Mg3N2 and Mg1Mg3N2) are pre-sented in the ESI (Fig. S2†). As is seen, imaginary frequenciesare not observed, showing that these structures are dynamically
stable at their ground states. Furthermore, AIMD simulationresults (refer to ESI Fig. S3† for details) also suggest that thethermal stability of the g-Mg3N2 can be stable at 300 K, even atthe highest metal-ion adsorption concentrations (Na3Mg3N2
and Mg1Mg3N2). The phonon dispersion curves and the AIMDsimulation results suggest that the g-Mg3N2 anode may havegood cycling stability.
3.3 The charge/discharge rate performance of g-Mg3N2
The speed of charge/discharge is an important consideration inbattery applications. The charge/discharge rate performance ofan anode material is determined by the electronic conductivityand metal-ions diffusivity. Here we considered both of them toevaluate the rate performance of g-Mg3N2. The electronic
density of states (DOS) of the g-Mg3N2 before and aer metal-ion adsorption are presented in Fig. 3. As is seen, the intrinsicelectronic structure of the g-Mg3N2 is undesirable semi-conductor (band gap 0.91 eV). However, upon differentconcentrations and species of metal-ions adsorption, the elec-tronic structures become metallic, as is shown in Fig. 3(b–f).This is benecial to improve the electronic conductivity duringthe charge/discharge process. In addition, we also found thatexcept Al, the more metal-ions are adsorbed, the higher inten-sity of the DOS around the Fermi level is observed. Above resultsindicate that more active electrons are created in the systemduring metal-ions adsorption process, showing better elec-tronic conductivity can be obtained at higher concentrations.
We had demonstrated that g-Mg3N2 have good electronicconductivity aer metal-ions adsorbed, next we will probe themetal-ions diffusions on the g-Mg3N2. Previous reports hadalready shown that the 2D materials possess high diffusion
Fig. 3 The total DOS of the intrinsic g-Mg3N2 (a) and its corresponding
27382 | RSC Adv., 2019, 9, 27378–27385
coefficient and low migration energy barriers.36,49,50 Since themetal-ions migration energy barrier is related to the metal-ionconcentration, we construct a 2 � 1 orthogonal supercell toreduce the inuence from periodic boundary conditions. Asdiscussed above, all metal-ion prefers to stay at the most ener-getically stable adsorption B-site. Therefore, we only considerthe different metal-ions migrations from one B-site to itsneighboring B-site.
Fig. 4(e) shows all possible pathways for single metal-ionmigration on the g-Mg3N2 surface. The name of pathway isranked by the length, the shortest distance pathway is namedPath-1, and the longest pathway is named Path-4. In fact, thePath-3 for Na-, K- and Ca-ion are nally become the same asPath-2 during the NEB calculation. For all positive chargedmetal-ions during migrations, the more positive charges theycarried, the stronger repulsive force they experienced. Thus, theNa+ and K+ migration energy barriers are much lower compared
M adsorbed states MxMg3N2 (b)–(f) with x ¼ 0.5 and 1 or 3.
Fig. 4 The metal-ions migration pathways on g-Mg3N2 (e) and the energy profiles along the optimized pathways (a–d) on g-Mg3N2. The blueand purples spheres represent the N atoms and different metal-ions adsorbed upon the Mg atom (B-site). The 2D metal-ions diffusion networksare illustrated by four colored arrows.
Paper RSC Advances
Ope
n A
cces
s A
rtic
le. P
ublis
hed
on 3
0 A
ugus
t 201
9. D
ownl
oade
d on
2/7
/202
2 6:
26:2
2 PM
. T
his
artic
le is
lice
nsed
und
er a
Cre
ativ
e C
omm
ons
Attr
ibut
ion-
Non
Com
mer
cial
3.0
Unp
orte
d L
icen
ce.
View Article Online
with Mg2+, Ca2+ and Al3+ during all migration pathways. Theobtained the migration energy barriers of all metal-ions areshown in Fig. 4(a–d). The lowest migration energy barriers forNa+, K+, Mg2+, Ca2+ and Al3+ are 315 meV in Path-1, 192 meV inPath-3 (actually Path-2), 828 meV in Path-1, 984 meV in Path-2and 937 meV in Path-1, respectively. In general, large ionradius would restrict the capacity and rate performance.However, our results show the charge of metal-ions, rather thanthe ion radius are the main factors to be responsible formigration energy barriers on g-Mg3N2. The ion radius of Li+,
Na+, K+, Mg2+, Ca2+ and Al3+ are 0.76, 1.02, 1.38, 0.72, 1.00 and0.54 A, respectively. Although the radius of the Al3+ is thesmallest of all metals, it suffered the highest barrier during themigration process. Furthermore, beneting from unique hoststructure, the migration energy barriers of Na- (315 meV) and K-ion (192 meV) on g-Mg3N2 are even lower than Li-ion (575meV).48 The energy barriers for Mg-, Ca- and Al-ion migrationare acceptable for a battery application considering their highcapacity and low cost. What's more, both Path-1 and Path-2 canmake a complete metal-ions diffusion network on the g-Mg3N2
surface, indicating that the charge/discharge rate of g-Mg3N2 issatisfactory for NLMIBs applications.
4. Summary and conclusions
In summary, we use density functional theory calculations forthe rst time to prove that 2D g-Mg3N2 can be applied as anappropriate anode material for NLMIBs. g-Mg3N2 possesseshigh theoretical storage capacity and maintain structuralstability during different metal-ions intercalation process. Theelectronic structure of g-Mg3N2 becomes metallic by metal-ionsintercalation. The satisfactory metal-ions migration energybarriers combined with complete 2D diffusion network ensuregood rate performance of g-Mg3N2 as anodes. Overall, wepredict theoretically that g-Mg3N2 has many advantages asa good anode material, such as good structural and electro-chemical stability, high metal-ions storage capacities, highelectronic conductivity, acceptable sequential intercalatedpotentials and metal-ions diffusion energy barriers, thus, it canbe applied as anode material for inexpensive and promisingNLMIBs.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work is supported by the Natural Science Foundation ofChina (NSFC, under Grant No. 51962010 and 11564016). Thecomputations were partly performed on TianHe-2(A) at theNational Supercomputer Center in Tianjin.
References
1 D. Larcher and J. M. Tarascon, Towards greener and moresustainable batteries for electrical energy storage, Nat.Chem., 2015, 7, 19–29.
2 J. B. Goodenough and K. S. Park, The Li-ion rechargeablebattery: a perspective, J. Am. Chem. Soc., 2013, 135, 1167–1176.
3 B. Dunn, H. Kamath and J. M. Tarascon, Electrical energystorage for the grid: a battery of choices, Science, 2011, 334,928–935.
4 J. M. Tarascon and M. Armand, Issues and challenges facingrechargeable lithium batteries, Nature, 2001, 414, 359–367.
5 M. A. Hannan, M. M. Hoque, A. Mohamed and A. Ayob,Review of energy storage systems for electric vehicleapplications: issues and challenges, Renewable SustainableEnergy Rev., 2017, 69, 771–789.
6 Y. M. Sun, N. Liu and Y. Cui, Promises and challenges ofnanomaterials for lithium-based rechargeable batteries,Nat. Energy, 2016, 1, 16071.
7 N. Yabuuchi, K. Kubota, M. Dahbi and S. Komaba, Researchdevelopment on sodium-ion batteries, Chem. Rev., 2014, 114,11636–11682.
27384 | RSC Adv., 2019, 9, 27378–27385
8 H. Y. Kang, Y. C. Liu, K. Z. Cao, Y. Zhao, L. F. Jiao, Y. J. Wangand H. T. Yuan, Update on anode materials for Na-ionbatteries, J. Mater. Chem. A, 2015, 3, 17899–17913.
9 Z. L. Jian, W. Luo and X. L. Ji, Carbon electrodes for K-ionbatteries, J. Am. Chem. Soc., 2015, 137, 11566–11569.
10 H. D. Yoo, I. Shterenberg, Y. Gofer, G. Gershinsky, N. Pourand D. Aurbach, Mg rechargeable batteries: an on-goingchallenge, Energy Environ. Sci., 2013, 6, 2265–2279.
11 Z. J. Zhao, J. P. Yao, B. Z. Sun, S. Y. Zhong, X. L. Lei, B. Xu andC. Y. Ouyang, First-principles identication of spinelCaCo2O4 as a promising cathode material for Ca-ionbatteries, Solid State Ionics, 2018, 326, 145–149.
12 M. C. Lin, M. Gong, B. G. Lu, Y. P. Wu, D. Y. Wang,M. Y. Guan, M. Angell, C. X. Chen, J. Yang, B. J. Hwangand H. J. Dai, An ultrafast rechargeable aluminium-ionbattery, Nature, 2015, 520, 324–328.
13 Y. Xie, Y. Dall'Agnese, M. Naguib, Y. Gogotsi,M. W. Barsoum, H. L. Zhuang and P. R. C. Kent,Prediction and characterization of MXene nanosheetanodes for non-lithium-ion batteries, ACS Nano, 2014, 8,9606–9615.
14 P. Xiang, X. F. Chen, W. T. Zhang, J. F. Li, B. B. Xiao, L. S. Liand K. Deng, Metallic borophene polytypes as lightweightanode materials for non-lithium-ion batteries, Phys. Chem.Chem. Phys., 2017, 19, 24945–24954.
15 X. Y. Deng, X. F. Chen, Y. Huang, B. B. Xiao and H. Y. Du,Two-Dimensional GeP3 as a high capacity anode materialfor non-lithium-ion batteries, J. Phys. Chem. C, 2019, 123,4721–4728.
16 M. J. Piernas-Munoz, E. Castillo-Martinez, O. Bondarchuk,M. Armand and T. Rojo, Higher voltage plateau cubicPrussian white for Na-ion batteries, J. Power Sources, 2016,324, 766–773.
17 X. F. Bie, K. Kubota, T. Hosaka, K. Chihara and S. Komaba, Anovel K-ion battery: hexacyanoferrate(II)/graphite cell, J.Mater. Chem. A, 2017, 5, 4325–4330.
18 S. W. Kim, D. H. Seo, X. H. Ma, G. Ceder and K. Kang,Electrode materials for rechargeable sodium-ion batteries:potential alternatives to current lithium-ion batteries, Adv.Energy Mater., 2012, 2, 710–721.
19 S. Okada, S. Sawa, M. Egashira, J. Yamaki, M. Tabuchi,H. Kageyama, T. Konishi and A. Yoshino, Cathodeproperties of phospho-olivine LiMPO4 for lithiumsecondary batteries, J. Power Sources, 2001, 97–98, 430–432.
20 X. D. Xiang, K. Zhang and J. Chen, Recent advances andprospects of cathode materials for sodium-ion batteries,Adv. Mater., 2015, 27, 5343–5364.
21 B. Xu, H. S. Lu, B. Liu, G. Liu, M. S. Wu and C. Y. Ouyang,Comparisons between adsorption and diffusion of alkali,alkaline earth metal atoms on silicene and those onsilicane: insight from rst-principles calculations, Chin.Phys. B, 2016, 25, 067103.
22 W. Luo, F. Shen, C. Bommier, H. L. Zhu, X. L. Ji and L. B. Hu,Na-ion battery anodes: materials and electrochemistry, Acc.Chem. Res., 2016, 49, 231–240.
23 M. Kazazi, Z. A. Zafar, M. Delshad, J. Cervenka andC. X. Chen, TiO2/CNT nanocomposite as an improved
anode material for aqueous rechargeable aluminumbatteries, Solid State Ionics, 2018, 320, 64–69.
24 E. Pomerantseva and Y. Gogotsi, Two-dimensionalheterostructures for energy storage, Nat. Energy, 2017, 2,17089.
25 H. Zhang, Ultrathin two-dimensional nanomaterials, ACSNano, 2015, 9, 9451–9469.
26 M. R. Lukatskaya, O. Mashtalir, C. E. Ren, Y. Dall'Agnese,P. Rozier, P. L. Taberna, M. Naguib, P. Simon,M. W. Barsoum and Y. Gogotsi, Cation intercalation andhigh volumetric capacitance of two-dimensional titaniumcarbide, Science, 2013, 341, 1502–1505.
27 Q. L. Sun, Y. Dai, Y. D. Ma, T. Jing, W. Wei and B. B. Huang,Ab initio prediction and characterization of Mo2Cmonolayeras anodes for lithium-ion and sodium-ion batteries, J. Phys.Chem. Lett., 2016, 7, 937–943.
28 J. H. Hou, K. X. Tu and Z. F. Chen, Two-dimensional Y2Celectride: a promising anode material for Na-ion batteries,J. Phys. Chem. C, 2016, 120, 18473–18478.
29 B. Mortazavi, O. Rahaman, S. Ahzi and T. Rabczuk, Flatborophene lms as anode materials for Mg, Na or Li-ionbatteries with ultra high capacities: a rst-principles study,Applied Materials Today, 2017, 8, 60–67.
30 B. Mortazavi, M. Shahrokhi, X. Y. Zhuang and T. Rabczuk,Boron-graphdiyne: a superstretchable semiconductor withlow thermal conductivity and ultrahigh capacity for Li, Naand Ca ion storage dagger, J. Mater. Chem. A, 2018, 6,11022–11036.
31 S. F. Lei, X. F. Chen, B. B. Xiao, W. T. Zhang and J. Liu,Excellent electrolyte wettability and high energy density ofB2S as a two-dimensional Dirac anode for non-lithium-ionbatteries, ACS Appl. Mater. Interfaces, 2019, 11, 28830–28840.
32 I. Demiroglu, F. M. Peeters, O. Gulseren, D. Cakir andC. Sevik, Alkali metal intercalation in MXene/grapheneheterostructures: a new platform for ion batteryapplications, J. Phys. Chem. Lett., 2019, 10, 727–734.
33 P. F. Liu, L. J. Zhou and L. M. Wu, A graphene-like Mg3N2
monolayer: high stability, desirable direct band gap andpromising carrier mobility, Phys. Chem. Chem. Phys., 2016,18, 30379–30384.
34 C. Y. Ouyang and L. Q. Chen, Physics towards nextgeneration Li secondary batteries materials: a short reviewfrom computational materials design perspective, Sci.China: Phys., Mech. Astron., 2013, 56, 2278–2292.
35 L. M. Zheng, Z. Q. Wang, M. S. Wu, B. Xu and C. Y. Ouyang,Jahn-Teller type small polaron assisted Na diffusion inNaMnO2 as a cathode material for Na-ion batteries, J.Mater. Chem. A, 2019, 7, 6053–6061.
36 H. W. Wang, M. S. Wu, X. L. Lei, Z. F. Tian, B. Xu, K. Huangand C. Y. Ouyang, Siligraphene as a promising anode
material for lithium-ion batteries predicted from rst-principles calculations, Nano Energy, 2018, 49, 67–76.
37 G. Kresse and J. Furthmuller, Efficient iterative schemes forab initio total-energy calculations using a plane-wave basisset, Phys. Rev. B: Condens. Matter Mater. Phys., 1996, 54,11169–11186.
38 P. E. Blochl, Projector augmented-wave method, Phys. Rev. B:Condens. Matter Mater. Phys., 1994, 50, 17953–17979.
39 G. Kresse and J. Joubert, From ultraso pseudopotentials tothe projector augmented-wave method, Phys. Rev. B:Condens. Matter Mater. Phys., 1999, 59, 1758–1775.
40 T. Bucko, J. Hafner, S. Lebegue and J. G. Angyan, Improveddescription of the structure of molecular and layeredcrystals: ab initio DFT calculations with van der Waalscorrections, J. Phys. Chem. A, 2010, 114, 11814–11824.
41 S. Grimme, Semiempirical GGA-type density functionalconstructed with a long-range dispersion correction, J.Comput. Chem., 2006, 27, 1787–1799.
42 H. J. Monkhorst and J. D. Pack, Special points for Brillouin-zone integrations, Phys. Rev. B: Solid State, 1976, 13, 5188–5192.
43 G. Henkelman and H. Jonsson, Improved tangent estimatein the nudged elastic band method for nding minimumenergy paths and saddle points, J. Chem. Phys., 2000, 113,9901–9904.
44 G. Henkelman, A. Arnaldsson and H. Jonsson, A fast androbust algorithm for Bader decomposition of chargedensity, Comput. Mater. Sci., 2006, 36, 354–360.
45 A. Togo and I. Tanaka, First principles phonon calculationsin materials science, Scr. Mater., 2015, 108, 1–5.
46 F. Zhou, M. Cococcioni, C. A. Marianetti, D. Morgan andG. Ceder, First-principles prediction of redox potentials intransition-metal compounds with LDA + U, Phys. Rev. B:Condens. Matter Mater. Phys., 2004, 70, 235121.
47 X. M. Zhang, J. P. Hu, Y. C. Cheng, H. Y. Yang, Y. G. Yao andS. Y. A. Yang, Borophene as an extremely high capacityelectrode material for Li-ion and Na-ion batteries,Nanoscale, 2016, 8, 15340–15347.
48 L. X. Xiong, J. P. Hu, S. C. Yu, M. S. Wu, B. Xu andC. Y. Ouyang, Density functional theory prediction ofMg3N2 as a high-performance anode material for Li-ionbatteries, Phys. Chem. Chem. Phys., 2019, 21, 7053–7060.
49 W. W. Luo, H. W. Wang, J. P. Hu, S. Q. Liu and C. Y. Ouyang,Curvature induced improvement of Li/Na storage in Ca2Nnanotubes, Appl. Surf. Sci., 2018, 459, 406–410.
50 J. P. Hu, C. Y. Ouyang, S. Y. A. Yang and H. Y. Yang,Germagraphene as a promising anode material forlithium-ion batteries predicted from rst-principlescalculations, Nanoscale Horiz., 2019, 4, 457–463.
